The present invention generally relates to electron beam exposure systems and more particularly to an electron beam exposure system for writing a pattern on an object by an electron beam.
With the requirement of increased integration density and reduced device size, use of the electron beam exposure systems is spreading in the fabrication of semiconductor integrated circuits. In the electron beam exposure system, an electron beam of rectangular or other suitable cross sections is used for writing a pattern on a semiconductor wafer while changing the size of the beam spot. Such an electron beam exposure system is particularly advantageous for patterning minute semiconductor devices, as the electron beam exposure system is capable of writing submicron patterns on the object. On the other hand, the electron beam exposure system has suffered from a problem of low throughput because of the feature pertinent to such a system that the pattern has to be written consecutively step by step by a single electron beam.
In order to avoid the foregoing problem of low throughput, a technique called block exposure has been proposed (IEEE TRANS. ON ELECTRON DEVICES, vol.ED-26, p.633, 1979), wherein a patterned mask is placed in the path of the electron beam between the electron gun and the object. The mask carries a number of patterned apertures that correspond to various basic patterns of semiconductor devices, and deflector systems are disposed at both sides of the mask. Thereby, the deflector at the upstream side of electron beam causes a deflection of electron beam away from an optical axis and directs the same to one of the patterned apertures. On the other hand, the deflector at the downstream side causes a deflection of the electron beam back to the original optical axis. Upon passage through the selected aperture, the electron beam is shaped as desired and hits the surface of the object, such as a semiconductor wafer, at a predetermined position after focusing and deflection caused by the usual electron lens and deflection systems.
FIG. 1 shows such an electron exposure system that carries out the block exposure.
Referring to FIG. 1, the electron exposure system includes an electron gun 14 that produces an electron beam 16. The electron beam 16 travels along a predetermined optical axis and experiences shaping upon passage through an aperture 18. After passing through the aperture 18, the electron beam is focused by an electron lens 20 at a point Pl located on the optical axis.
At the point P1, there is provided an electrostatic deflector 22 that causes a deflection of the electron beam 16 away from the optical axis in response to a control signal applied thereto. At the downstream side of the deflector 22, there is provided a mask 10 in which a number of patterned apertures 12-1 - 12-5 are formed, and the deflector 22 causes the deflection of the electron beam 16 to one of the apertures on the mask 10. Upon passage through the selected aperture, the electron beam 16 experiences shaping for the second time and thus, the electron beam exiting from the mask 10 has a desired cross section.
In correspondence to the mask 10, there is provided an electron lens 28 such that the optical axis of the lens 28 coincides with the optical axis of the electron beam, and the lens 28 focuses the electron beam passing therethrough. More specifically, the electron beam 16 that has exited from the mask 10 is deflected and focused on a point P2 located on the original optical axis of the beam 16. Further, the electron beam is demagnified by a lens 30 and deflected by the deflectors 34 and 36 provided immediately above a semiconductor wafer 38. Thereby, the electron beam 16 hits the predetermined part of the surface of the wafer 38 with a desired cross section.
In the foregoing conventional apparatus, however, there exists a problem in that, associated with the focusing action of the electron lens 28 that deflects back the electron beam to the point P2 on the optical axis, the electron beam tends to experience deformation by various aberration effects that are caused by the electron lens. It should be noted that the path length of the electron beam changes depending on what aperture on the mask 10 is selected. The effect of aberration appears strongly particularly when the aperture which is far from the optical axis is selected since as the electron beam is deflected by a large angle and then deflected back by also a large angle in such a case.
In order to eliminate the foregoing problems, the applicant of the present invention has previously proposed an electron beam exposure system wherein an incidence side electron lens and an exit side electron lens are disposed respectively at the upstream side and the downstream side of the mask, and the electron beam passes through the aperture in the mask with an angle perpendicular to the plane of the mask. Between the incidence side electron lens and the mask, there is provided an incidence side deflector system for shifting the path of the electron beam parallel to the optical axis. Further, between the mask and the exit side electron lens, there is provided an exit side deflector system for shifting back the electron beam back to the original optical path.
FIG. 2 shows the electron beam exposure system described above.
Referring to FIG. 2, there is provided an electron gun 68 that produces an electron beam 70. The electron beam 70 travels along a predetermined optical axis 90 and passes through a beam shaping aperture 72. Thereby, the electron beam 70 experiences beam shaping and passes through an electron lens 74 subsequently. The electron lens 74 focuses the electron beam 70 on a point P1 located on the optical axis 90 and a minute adjustment of the electron beam is achieved by a deflector 76.
The electron beam is then passed through another electron lens 78 where it is converted to a parallel electron beam, and the parallel electron beam thus formed hits a mask 40 substantially perpendicularly. The mask 40 is formed with a number of patterned apertures in correspondence to the mask 10 of the system of FIG. 1, and the electron beam is shaped as desired upon passage through a selected aperture. The electron beam, thus passed through and shaped by the mask 40, is then received by another electron lens 92 located at the downstream side of the electron beam 40 and focused on a point P2 located on the optical axis 90. The mask 40 carries a large number of patterned apertures that which differ from each other and which are arranged in aperture groups, and these aperture groups are selectively placed into the area through which by the electron beam passes by a mask drive unit 104 which moves the mask 40 in a direction perpendicular to the optical path 90.
In order to effect the desired shifting of the electron beam, the system of FIG. 2 employs a pair of deflectors 80 and 82 provided between the lens 78 and the mask 40 wherein the deflector 80 deflects the electron beam away from the optical axis 90 and the deflector 82 deflects the electron beam thus deflected back to a path parallel to the original optical path 90. Further, there are provided deflectors 84 and 86 between the mask 40 and the lens 92 such that the electron beam, after passing through the mask 40, is deflected toward the optical axis 90 by the deflector 84 and the electron beam thus deflected by the deflector 84 is further deflected by the deflector 86 such that the electron beam returns to the original optical path 90. The foregoing lens 92 focuses the electron beam, after experiencing the deflection of the deflector 86, on the point P2. Further, there is provided a control unit 88 which supplies control signals to the deflectors 80, 82, 84 and 86, and in response to the control signals, the foregoing deflection of the electron beam occurs.
The electron beam thus obtained at the point P2 is shaped according to the pattern of the selected aperture on the mask 40 and is focused on the surface of a wafer 102 after passing through the usual electron optical system that includes lenses 94 and 98 as well as deflectors 100. Between the lens 94 and lens 98, there is provided an aperture or pinhole 96 for proper alignment of the electron optical system. More specifically, the pinhole 96 has a limited diameter of about 100 .mu.m and allows the passage of an electron beam there through only when the electron beam has traveled along the optical axis 90 through the electron lenses 94 and 98. The role of the pinhole 96 will be described later in relation to the present invention.
In this electron exposure system, it should be noted that the electron beam passes through the aperture on the mask 40 in the direction substantially perpendicular to the plane of the mask. Further, the electron beam passes only through the central part of the lenses 78 and 92. Thus, one can eliminate the problem of aberration caused by the electron lens by using this electron exposure system.
In such an electron exposure system known commonly as the block exposure system, it is necessary to calibrate the deflectors 80 and 82 at the upstream side of the mask 40 as well as the deflectors 84 and 86 at the downstream side. Such a calibration includes two types of calibrations, i.e., a) calibration about the mutual relationship of the deflection angles caused by the deflectors 80-86; and b) calibration about the absolute magnitude of deflection caused by the deflectors 80-86. Here, the calibration a) establishes a mutual relationship between the control signals applied to the deflectors 80-86 such that the electron beam deflected away from the optical axis 90 by the deflector 80 returns to the optical axis 90 again after deflection by the deflector 86. The calibration b) on the other hand determines the absolute magnitude of the control signals that are supplied to the deflectors 80-86 for the desired deflection. Such a calibration process is generally time consuming and decreases the productive throughput of the electron exposure.